In the field of projection printing and especially in the semiconductor industry and in the manufacture of integrated circuits the use of pellicles to shield a photomask from airborne particulate matter has gained increasing acceptance and use because of the multitude of benefits derived from their use. The employment of a pellicle for projection printing is described, for example, in U.S. Pat. No. 4,131,363, issued Dec. 26, 1978 to Shea et al and the article of Spivack et al entitled, "Evaluation of Parylene and Other Pellicles as Beam Splitters", Electro-Optical System Design Corp., (1973) pages 362-370, which are herewith incorporated by reference. The use of such pellicles in projecting printing has resulted in a reduction in mask and printing defects, increased mask reticle life, reduced aligner downtime, increased mask quality and increased device and reticle yield. Materials employed in the prior art and suitable for the production of films utilizable as pellicle films include polymer films of polyoxyethylene terephthalate, nitrocellulose and parylene.
Prior art polymer films other than polyvinyl butyral while finding usefulness in projection printing systems as pellicle films have been severely limited in their usefulness. For example, while pellicles of such polymer films are useful in the near UV region (wavelengths 340-436 nm) and some usefulness in the mid UV region (wavelengths 280-340 nm) none has been acceptable for use in the deep UV region (wavelength 200-280 nm). Prior art pellicle films, while providing acceptable properties for use in the near and mid UV regions, have not provided acceptable light transmission properties in the deep UV region and thereby cannot be utilized in the deep UV region.
Additionally, although the prior art pellicle films have found usefulness in the near and mid UV regions, their performance has not been entirely satisfactory. Among the many factors detracting from their usefulness in these regions is the fact that their light transmission properties are not substantially 100% at the projection wavelengths but rather are generally 99% or less. Also, film tensile strength of the pellicle films has generally been in the range of 3000-5000 psi and the need for a film of increased tensile strength to increase the pellicle film life is desirable. Additionally, the percent of undesirable light scattering in said prior art pellicle films has generally been in the range of 0.3% at 365 nm. A film of decreased light scattering is highly desirable. Also with the prior art pellicle films, the percent etaloning, namely the difference between peak and minimum transmission, has only been as low as about 18% and it is of great advantage to reduce such etaloning as much as possible to increase the average light transmission properties of the pellicles.
It is highly desirable that a pellicle film be available which is useful in the deep UV region and which is as good or better in performance characteristics and/or physical properties as has been the case with pellicle films used in the mid or near UV regions. Moreover, it is also highly desirable that such pellicle film be useful in all three UV regions and that the performance characteristics and/or physical properties of such pellicle film in the mid and near UV regions be improved in whole or in part compared to the performance characteristics and/or physical properties of the materials heretofore employed in the prior art for pellicle films in the mid or near UV regions.
It is known that not all polyvinyl butyral resin films provide the necessary optical properties to function as a pellicle for projection printing. It is further desirable that polyvinyl butyral pellicles be free of any impurities which may interfere with its transmission of light. A common problem with polyvinyl butyral pellicles resides in the presence of a carbonyl group which is believed to be derived from the vinyl acetate found in the precursor materials. In addition, the solvent systems which have been previously used with the polyvinyl butyral resins to form the pellicles have not resulted in the most efficient solvent system for the minimization of impurities. Surprisingly, applicant has discovered that dimethylformamide which contains a carbonyl group is an effective solvent.
Pellicles may be prepared by any of the conventional methods of casting, float casting or rotation spinning. Generally, the polymer/solvent mixtures constitute from about 5% to about 30% by weight of polymer. Leveling agents such as silicone fluids may be added.
Castable solutions of the polyvinyl butyral of the invention may be made depending upon the method used to make the pellicle. Generally a casting solution of 10-15% by weight of solids will result in film formed by rotation spinning between 2000 and 3000 rpm of about 1.2-2.0 microns.
U.S. Pat. No. 4,482,591 to Irl E. Ward relates to polyvinyl butyral pellicles which are formed from specific polyvinyl butyral resins possessing specified hydroxy, acetate and butyral content, namely, less than 13%, less than 2% and more than 88% respectively. This patent specifically teaches the utilization of a polyvinyl butyral derived from a solvent system comprising 2-ethoxyethanol and tetrahydrofurfuryl alcohol or methyl ethyl ketone.